The present invention relates to an air-fuel ratio control system for an engine.
When a three-way catalyst of a catalytic converter is not deteriorated, an output of a downstream O2 sensor disposed on a downstream side of the catalytic converter has a long inversion period, due to an oxygen storage function of the three-way catalyst. When the three-way catalyst is deteriorated, however, the inversion period of the output of the downstream O2 sensor becomes shorter (approaching an inversion period of an output of an upstream O2 sensor disposed on an upstream side of the catalytic converter). Whether or not the three-way catalyst is deteriorated can be diagnosed in accordance with a ratio of the inversion period of the downstream O2 sensor output to the inversion period of the upstream O2 sensor output.
However, in the case of an engine having two cylinder groups provided with respective three-way catalytic converters in respective exhaust passages, and two upstream oxygen sensors for sensing the air fuel ratios on the upstream side of the catalytic converters to feedback-control the air fuel ratios of the two cylinder groups individually, the above-mentioned diagnosis of the three-way catalyst requires downstream O2 sensors for the two catalytic converters to the disadvantage of cost. In a diagnostic system employing only one downstream O2 sensor in a common exhaust passage into which the two exhaust passages from the two cylinder groups merge, the accurate diagnosis is possible only when the rich-lean air-fuel ratio variations of the two cylinder groups are in phase. If the rich-lean air-fuel ratio variations of the two cylinder groups are out of phase or in opposition, the rich side of one cylinder group and the lean side of the other cylinder group cancel each other, and hence the output waveform of the downstream O2 in the common exhaust passage becomes flatter with little inversion, irrespective of deterioration or non-deterioration of the three-way catalyst.
Japanese-Patent Examined Publication No. 8(1996)-6624 describes an air-fuel ratio control system for controlling the air fuel ratios of two cylinder groups in accordance with an output of one of upstream O2 sensors when diagnosis is required to detect deterioration of the three way-catalytic converters.
However, this conventional system might decrease the effect of exhaust gas purification by leaving one cylinder group uncontrolled during the diagnosis. The diagnosis is performed at the cost of the emission control performance.
In this case, a system called a double O2 sensor system can control the air-fuel ratio in the common exhaust passage at the stoichiometric level with a downstream O2 sensor whose output is used to modify the air-fuel ratio feedback correction coefficient based on the output of the upstream O2 sensor. This system can ensure good exhaust emission purification by adding a third three-way catalytic converter. However, the control system cannot always hold both of the air-fuel ratios of the first and second cylinder groups at the stoichiometric ratio, so that it is difficult to maintain the efficiency of the three-way catalyst of each cylinder group at a satisfactory level. If, for example, the air-fuel ratio of the first cylinder group is controlled at the stoichiometric level by the feedback control based on the output of the oxygen sensor for the first cylinder group, but the air-fuel ratio of the second cylinder group is shifted to the rich side, then the air-fuel ratio in the common exhaust passage is on the rich side and the double oxygen sensor system acts to shift the air-fuel ratios of both cylinder group toward the lean side. As a result, the air-fuel ratio of the first cylinder group becomes slightly lean whereas the air-fuel ratio of the second cylinder group becomes slightly rich. The control continues until the air-fuel ratio in the common exhaust passage becomes equal to the stoichiometric air-fuel ratio. This is true of another situation in which the air-fuel ratio of the second cylinder group is shifted to the lean side.
Moreover, in this double O2 sensor system, the speed of the correction based on the output of the downstream O2 sensor is generally low. Therefore, it requires a considerable time to secure the exhaust gas mixture purifying efficiency with the three-way catalyst in the common exhaust passage. During this, the exhaust emission control can be poor.
It is an object of the present invention to provide air-fuel ratio control technique for synchronizing air-fuel ratio variations of two cylinder groups, and simultaneously without costing the exhaust emission control efficiency in both of two cylinder groups.
1) There is provided an air-fuel ratio control system for an engine according to the present invention. This air-fuel ratio control system comprises; a first cylinder group; a second cylinder group; a first catalytic converter disposed in a first exhaust passage from the first cylinder group; a second catalytic converter disposed in a second exhaust passage from the second cylinder group; a first air-fuel ratio sensor sensing an air-fuel ratio of an exhaust gas mixture flowing into the first catalytic converter; a second air-fuel ratio sensor sensing an air-fuel ratio of an exhaust gas mixture flowing into the second catalytic converter; and a controller calculating a first air-fuel ratio feedback correction coefficient in accordance with an output of the first air-fuel ratio sensor, feedback-controlling an air-fuel ratio of the first cylinder group by using the first air-fuel ratio feedback correction coefficient, determining whether a predetermined phase synchronization request is present for synchronizing air-fuel ratio variation of the first and second cylinder groups, measuring a rich time and a lean time in the air-fuel ratio variation of the second cylinder group in accordance with an output of the second air-fuel ratio sensor to determine a second cylinder group""s ratio between the rich time and the lean time when the synchronism request is present, calculating a correction quantity to bring the second cylinder group""s ratio closer to a target ratio when the synchronization request is present, determining a modified coefficient by modifying the first air-fuel ratio feedback correction coefficient with the correction quantity, and feedback-controlling the air-fuel ratio of the second cylinder group by using the modified coefficient as a second air-fuel ratio feedback correction coefficient when the in-phase request is present.
2) There is provided an air-fuel ratio control process for an engine according to the present invention. This air-fuel ratio control process comprises; ascertaining a sensed first air-fuel ratio of an exhaust gas mixture flowing into a first catalytic converter; ascertaining a sensed second air-fuel ratio of an exhaust gas mixture flowing into a second catalytic converter, calculating a first air-fuel ratio feedback correction coefficient in accordance with the sensed first air-fuel ratio, to feedback-control an actual air-fuel ratio of a first cylinder group by using the first air-fuel ratio feedback correction coefficient; determining whether a predetermined phase synchronization request is present for synchronizing air-fuel ratio variation of first and second cylinder groups; measuring a rich time and a lean time in the air-fuel ratio variation of the second cylinder group in accordance with the sensed second air-fuel ratio to determine a second cylinder group""s ratio between the rich time and the lean time when the synchronization request is present; calculating a correction quantity to bring the second cylinder group""s ratio closer to a target ratio when the synchronization request is present; and determining a modified coefficient by modifying the first air-fuel ratio feedback correction coefficient with the correction quantity, to feedback-control the air-fuel ratio of the second cylinder group by using the modified coefficient as a second air-fuel ratio feedback correction coefficient when the synchronization request is present.
3) There is provided an air-fuel ratio control apparatus for an engine according to the present invention. This air-fuel ratio control apparatus comprises; means for calculating a first air-fuel ratio feedback correction coefficient in accordance with an output of a first air-fuel ratio sensor; means for feedback-controlling an air-fuel ratio of a first cylinder group by using the first air-fuel ratio feedback correction coefficient; means for determining whether a predetermined phase synchronization request is present for synchronizing air-fuel ratio variation of first and second cylinder groups; means for measuring a rich time and a lean time in the air-fuel ratio variation of the second cylinder group in accordance with an output of a second air-fuel ratio sensor to determine a second cylinder group""s ratio between the rich time and the lean time when the synchronization request is present; means for calculating a correction quantity to bring the second cylinder group""s ratio closer to a target ratio when the synchronization request is present; means for determining a modified coefficient by modifying the first air-fuel ratio feedback correction coefficient with the correction quantity; and means for feedback-controlling the air-fuel ratio of the second cylinder group by using the modified coefficient as a second air-fuel ratio feedback correction coefficient when the synchronization request is present.